1. Field of the Invention
The invention relates to varistors and more particularly to varistors having improved energy absorption characteristics.
2. Description of the Prior Art
Varistors using elements formed of ZnO in combination with other elements are the basis for all modern varistor designs. During service, the varistors continuously draw a small leakage current. During transient voltage conditions which increase the voltage stress to which the varistor is subjected, the current increases. This converts a sufficient amount of electrical energy to heat, to limit the amplitude of voltage surges to an acceptable value. However, the electrical behavior of varistors is very sensitive to temperature. For example, at a high temperature the current at a constant voltage stress typically irreversibly increases exponentially.
In service, typical varistors are operated at a voltage stress in the region of 0.4 to 0.8 E.sub.0.5. (E.sub.0.5 is the voltage stress at which 5 ma. per cm.sup.2 flows through the varistor). During surge conditions, which increase the voltage stress to which the varistor is subjected, energy absorption increases causing the temperature of the varistor to increase in direct relation to the amount of energy absorbed. A typical relation is a temperature increase in the order of 10.degree. C. per 40 J/cc absorbed. Since the voltage surge is transient, the varistor is heated but the voltage stress returns to normal after a short time interval. However, the leakage current will have increased as a result of the transient, due to the additional heating caused thereby. Each subsequent transient which results in an increase in the temperature of the varistor further increases the leakage current. These incremental increases in current are an important factor determining the life of a varistor.
The varistor action is related to the defect state at the boundaries of the ZnO grains. These defects are due to the levels and type of additives (materials other than ZnO) included in the mixture used to make the varistor disc. The defect structures are in a semi-equilibrium state and are subject to increases in mobility as the temperature rises. Consequently, even though the varistor may be stable indefinitely at normal operating temperature and voltage stress, at an elevated temperature these defects can be unstable, causing the varistor current to increase and thereby increasing the amount of electrical energy absorbed, further heating the varistor. This condition rapidly becomes uncontrollable, resulting in an exponentially increasing current and failure of the varistor.